The deaths of seven CIA agents and a Jordanian intelligence officer in a Dec. 30 suicide bombing--the agency's worst such incident in more than 25 years--were shocking enough. Even more alarming were reports that the attacker, Humam Khalil Abu-Mulal al-Balawi, 32, was a double agent who had been providing information on al-Qaeda to U.S. and Jordanian officials for at least a year. Analysts say that in addition to straining ties between intelligence communities in Washington and Amman, the incident could hinder progress in hunting down terrorists, including Ayman al-Zawahiri, al-Qaeda's No. 2. Some officials...